1. Field of the Invention
The present invention relates to a high power amplifier mainly used at a cellular phone base station, and a balun circuit for the high power amplifier.
2. Related Art of the Invention
A high power amplifier capable of propagating several tens of watts to several hundreds of watts has recently been used at a digital mobile communication base station. This kind of high power amplifier comprises a plurality of push-pull amplifiers combined in parallel to obtain high power, wherein each push-pull amplifier comprises two same transistors connected in parallel and supplied with signals 180 degrees out of phase with each other. The explanation of the push-pull amplifier has been given on pages 113 to 116 of "Radio Frequency Transistors" written by Norm Dye and Helge Granberg, published by Butterworth/Heinemann. Therefore, no detailed explanation is given here. This power amplifier circuit requires a power splitter/combiner circuit and baluns at each of the input and output of the circuit.
A conventional high power amplifier will be described below referring to FIG. 11. In FIG. 11, the numeral 501 designates a n-way power splitter, the numeral 502 designates a n-way power combiner, and the numerals 503, 504 designate n baluns, and the numeral 505 designates n pairs of push-pull amplifiers. A power splitter/combiner circuit used in this configuration will be described below referring to FIGS. 12A, 12B and 13. FIGS. 12A, 12B and 13 show the configurations of Wilkinson power splitter circuits. FIG. 12A shows a general Wilkinson power splitter circuit. The numeral 601 designates an input terminal, the numeral 602 designates n quarter-wavelength lines, the numeral 603 designates n isolation resisitors, and the numeral 604 designates n output terminals. FIG. 12B shows a tree configuration of two-way splitters capable of being configured as a plane circuit. The numeral 605 designates an input terminal, the numeral 606 designates two quarter-wavelength lines, the numeral 607 designates an isolation resisitor, the numeral 608 designates four quarter-wavelength lines, the numeral 609 designates two isolation resisitors, and the numeral 610 designates four output terminals. In addition, FIG. 13 shows an asymmetric power splitter type. The numeral 611 designates an input terminal, the numerals 612, 613, 616 and 617 designate quarter-wavelength lines having characteristic impedances different from one another, the numerals 614 and 618 designate isolation resisitors, and the numeral 615, 619 and 620 designate impedance transformer circuits. The explanation of Wilkinson power combiner circuit has been given on pages 205 to 210 of "Foundations of Microwave Circuits and Applications thereof" written by Yoshihiro Konishi, published by Sogo Denshi. Therefore, no detailed explanation is given here.
FIG. 14 shows the configuration of a conventional balun. The numeral 701 designates an unbalanced terminal, the numeral 702 designates a quarter-wavelength coaxial line having a characteristic impedance of 50.OMEGA., the numeral 703 designates an in-phase output terminal, and the numeral 704 designates an opposite-phase output terminal. The explanation of the balun has been given on pages 179 to 181 of "Radio Frequency Transistors" written by Norm Dye and Helge Granberg, published by Butterworth/Heinemann. Therefore, no detailed explanation is given here.
On the basis of the above-mentioned configurations, the conventional high power amplifier has obtained high power for transmission at a base station by combining a plurality of output powers of push-pull amplifiers.
Although the Wilkinson power splitter circuit shown in FIG. 12A can evenly split power into n-way port at one time, the circuit cannot be attained as a plane circuit. For this reason, the configurations shown in FIGS. 12B and 13 are used generally. The configuration shown in FIG. 12B, however, loads to large loss in splitter/combiner due to a long transmission path. In addition, the configuration also has a problem of low versatility, because power can be split and propagated only to 2.sup.n paths. Furthermore, the configuration shown in FIG. 13 has a problem of unbalanced power splitting, because transmission paths length are different one output to another. Moreover, the balun shown in FIG. 14 is large in circuit size because of the use of the coaxial line, thereby having a problem of difficulty in miniaturization.